The vapor pressures of sulfur in equilibrium with various compositions within the Cu−Fe−S system were measured by a molecular absorption technique. Measurements were made as functions of temperature for the single phase compositions CuFeS1.62, CuFeS1.70, CuFeS1.80 and CuFeS1.90, for the two-phase fields bornite+chalcopyrite and pyrrhotite+chalcopyrite, and for the three-phase field chalcopyrite+bornite+pyrite. Statistical mechanical equations are derived and used to evaluate the data. Chalcopyrite highly deficient in sulfur behaves similarly to an ideal mixture of Cu2S and FeS with a random distribution of the constituent cations. Calculated values are given for the Gibbs energy of formation of chalcopyrite at 973 K for compositions CuFeS1.62 to CuFeS2, and the entropies and Gibbs energies of formation for CuFeS2(s) from 800 to 1000 K. Standard entropies, enthalpies, and energies are derived: S0 CUFeS 2 (298)=32 e.u., ΔH0 CuFeS 2,f(298)=−42,116 cal, ΔG0 CuFeS 2 f (298)=−42,800 cal.